Fuel-fired appliances must meet numerous safety standards. For example, current generation water heaters must be flammable vapor ignition resistant, or FVIR. A common approach to constructing an FVIR water heater is to pass all of the combustion air through a flame arrestor prior to mixing with the supplied fuel. In this manner, the fuel burner is isolated from the environment, reducing the risk of ignition of flammable vapors that could be in the environment. Flame arrestors can become fouled from lint, dirt, and oil (LDO) during the appliances operational lifetime. This flame arrestor fouling can starve the combustion process for air, causing carbon monoxide to be produced. Due to the risk of carbon monoxide production, standards also require that fuel-fired appliances be equipped with some means of shutting the appliance off if the combustion process may be producing excessive carbon monoxide. Some water heaters include shut off mechanisms that are triggered by increased operating temperature, which is one indication that the combustion air is being limited.
Some new cleaner fuel burning appliances have burner systems in which all the needed combustion air is provided through the main burner. Secondary combustion chamber relief openings are provided to enhance combustion stability and emissions performance. Because of airflow and thermal balances, this style of appliance will exhibit a decrease in operational temperatures in the event that the burner becomes fouled, making previously known carbon monoxide shut-off mechanisms that are triggered by increased operational temperatures ineffective.